<Overall Arrangement of Rotary Compressor>
With reference to FIGS. 3 to 5, an overall arrangement of a rotary compressor will be described. FIG. 3 is a vertical cross-sectional view showing an overall arrangement of a rotary compressor, FIG. 4 is a cross-sectional view taken along a line IV-IV in a direction of arrows in FIG. 3, and FIG. 5 schematically illustrates a flow of compressed gas inside a muffler.
This rotary compressor includes a casing 1, and this casing 1 has a cylindrical shape with its inside being sealed. A compression element 4 is provided on a lower end side, and a drive element 3 for actuating compression element 4 is provided thereabove. A discharge pipe 2 is provided in an upper portion of casing 1. An oil storage 21 for storing a lubricant O is formed in a lower end portion of casing 1, and a storage space 22 for storing compressed gas is formed in other space.
<Compression Element 4>
Compression element 4 includes a cylinder 9 that includes a cylinder chamber 9a having a circular transverse cross-sectional shape, and on both upper and lower surfaces of this cylinder 9, a front head 12 having a boss-shaped bearing portion 12a at its center and a rear head 13 also having a boss-shaped bearing portion 13a at its center are fastened with a plurality of through bolts (not shown), thus putting cylinder chamber 9a in a sealed state. A piston 11 is disposed in cylinder chamber 9a of cylinder 9. This piston 11 is eccentrically disposed in cylinder chamber 9a by a roller 10 of a crankshaft 7.
<Drive Element 3>
Drive element 3 includes an electric motor constituted of a stator 5 and a rotor 8, with stator 5 being fixedly supported to an inner wall surface of casing 1. Rotor 8 is concentrically disposed on the inner side of stator 5 with a prescribed gap 6 in a circumferential direction. An upper half portion of crankshaft 7 is mounted inside rotor 8 around a shaft center to rotate together, and a lower half portion of crankshaft 7 is rotatably supported by fitting and insertion by both bearing portions 12a and 13a of respective front head 12 and rear head 13. A discharge port 14 provided in front head 12 is provided with a leaf-spring shaped discharge valve 15, to prevent backflow of the compressed gas to cylinder chamber 9a. 
<Muffler Structure>
A first muffler 16 provided to cover discharge port 14 and surround crankshaft 7 and a second muffler 17 provided to cover first muffler 16 and surround crankshaft 7 are provided around bearing portion 12a of front head 12. A rotary compressor having such a double muffler structure is disclosed in Japanese Patent Laying-Open No. 5-0133377.
As shown in FIG. 4, first muffler 16 is provided with a first muffler crankshaft hole 16h through which crankshaft 7 and bearing portion 12a of front head 12 surrounding crankshaft 7 pass, and first muffler discharge outlets 16a, 16b disposed symmetrically in a direction displaced from a position of discharge port 14 by 90 degrees around crankshaft 7. Further, second muffler 17 is provided with a second muffler crankshaft hole 17h through which bearing portion 12a of front head 12 surrounding crankshaft 7 passes, and second muffler discharge outlets 17a, 17b disposed symmetrically in a direction displaced from the positions of first muffler discharge outlets 16a, 16b by 90 degrees around crankshaft 7.
As shown in FIG. 5, the compressed gas discharged from discharge port 14 passes through first muffler discharge outlets 16a, 16b of first muffler 16, and successively passes through second muffler discharge outlets 17a, 17b of second muffler 17. Accordingly, a two-stage muffling effect by the mufflers (particularly lowering in sound of 800 Hz band) can be expected.
Here, an outer shape of second muffler 17 has a shape of a cup as shown in FIG. 3, and a side surface thereof is constituted mostly of an inclined region. FIG. 6 shows a plan view of second muffler 17, where the inclined region is indicated with hatched lines. Second muffler discharge outlets 17a, 17b are provided in positions facing each other, and openings thereof are formed to include the inclined portion. This is because if second muffler discharge outlets 17a, 17b are provided to avoid the inclined region, second muffler discharge outlets 17a, 17b will have a reduced opening diameter, resulting in an increased discharge pressure loss.
When second muffler discharge outlets 17a, 17b are formed to include the inclined region in this manner, second muffler discharge outlets 17a, 17b open partially toward casing 1. As a result, as shown in FIG. 7 which is a cross-sectional schematic view, the compressed gas discharged from second muffler discharge outlets 17a, 17b is discharged toward casing 1 (a direction of an arrow G1 in the diagram).
Here, the compressed gas discharged from second muffler discharge outlets 17a, 17b includes not only gas but also lubricant, and the compressed gas and the lubricant are separated from each other while moving to discharge pipe 2 provided in the upper portion of casing 1. Then, as shown in FIG. 7, the compressed gas separated from the lubricant is discharged from discharge pipe 2 (a direction of an arrow G2 in the diagram). On the other hand, the lubricant separated from the compressed gas is returned along the inner wall surface of casing 1 to oil storage 21 (a direction of an arrow O1 in the diagram).
As described above, however, since the compressed gas discharged from second muffler discharge outlets 17a, 17b is discharged toward casing 1 (the direction of arrow G1 in the diagram), the direction in which the compressed gas is discharged (G1 direction) and the direction in which the lubricant is returned (O1 direction) will collide with each other on the inner wall surface of casing 1. Accordingly, there is apprehension that the return of the lubricant inside casing 1 may be blocked.